The preparation of nitric acid generally starts with oxidizing ammonia with air over a catalyst, so as to produce nitric oxide (NO). The process continues with oxidizing NO to nitrogen dioxide (NO2). Nitrogen dioxide will form nitric acid with water. The corresponding reaction equations are as follows:4NH3+5O2→4NO+6H2O  (1)2NO+O2→NO2  (2)3NO2+H2O→2HNO3+NO  (3)
In the nitric acid production process, the formation of the acid from nitrogen dioxide occurs in two instances. The nitrogen oxides gas stream obtained from the oxidation of ammonia, which also comprises water as formed in the oxidation of ammonia, is subjected to condensation. Thereby a part of the nitrogen dioxide forms nitric acid. Further, the remaining nitrogen dioxide is subjected to absorption in water, so as to complete the formation of nitric acid. In order for a maximum amount of the nitrogen dioxide NO2 obtained to be absorbed by water, the absorption is generally effected at elevated pressure, preferably at pressures between 4 and 14 bar. In the absorption section also oxygen is added, such as in the form of air, in order to allow nitrogen oxide (NO) to form nitrogen dioxide. Non absorbed nitrogen oxides (NO and/or NO2, generally indicated as NOx) as well as N2O, formed as by product of the oxidation of ammonia, result in a tail gas which is purified and vented into the atmosphere in conventional manner via a stack. A general reference is Behr, A., 2002. Ullmann's Encyclopedia of Industrial Chemistry, Vol. 6, Wiley-VCH, Weinheim.
The oxygen required for the conversion of the ammonia used as the raw material is generally supplied in the form of air. For the purpose of supply, the process air is compressed in a compressor and brought to a pressure appropriate for the oxidation reaction. Optionally, the formed NOx gas can be compressed by a second compressor to an appropriate pressure for the absorption reaction.
Modern nitric acid plants are operated under pressure in order to achieve higher acid concentrations and higher absorption rates of NOx (i.e. better efficiencies) in the absorption. A distinction is made between two-pressure and one-pressure plants. In the one-pressure process, both the combustion and the absorption are performed at moderate pressure (<8 bar) or high pressure (>8 bar). In the two-pressure (dual pressure) plants, the gas production, i.e. the production of the nitrous gases by oxidation of ammonia, is effected under a pressure of about 4 to 6 bar, and the absorption of the nitrous gases thus obtained with water to give nitric acid typically at 8 to 12 bar. The pressure is generated using compressors which are driven by means of a gas and/or steam turbine or electric motor. A gas turbine is preferably operated utilizing the energy of the off-gas of the nitric acid plant using the pressure applied by the at least one compressor.
A plant for the production of nitric acid based on the aforementioned oxidation of ammonia, generally comprises the following sections:                a burner section (i.e. a reactor comprising a combustion chamber) wherein a gas mixture of ammonia and air are reacted so as to form a burner gas stream; the burner gas stream comprises nitrogen dioxide formed by oxidation of nitrogen oxide, and further an amount of non-oxidized nitrogen oxide, nitrous oxide (N2O) as a by-product, as well as possibly N2 as a by-product, and water obtained from the oxidation of ammonia;        a gas cooling section wherein the burner gas stream is subjected to cooling, so as to form a cooled burner gas stream, and generally raising steam;        a condensation section wherein said cooled burner gas stream is subjected to condensation, so as to form a nitric acid solution and an uncondensed nitrogen oxides (NOx) gas stream;        an absorption/oxidation section; therein the nitrogen oxides gas stream is subjected to absorption in water so as to form a raw nitric acid product stream and a nitrogen oxides tail gas comprising NOx (nitrogen oxide and nitrogen dioxide) nitrous oxide (N2O), and typically also N2 and any excess oxygen; preferably in this section the gas is provided with a residence time allowing NO to be further oxidized into NO2;        a tail gas treatment section, comprising a tail gas heating zone, a nitrogen oxides (NOx and N2O) gas removal zone, an expansion zone and a heating zone, wherein the nitrous gas is subjected to purification so as to form a purified tail gas, said purified tail gas being subjected to expansion. Thereby the expanded purified tail gas is vented into the atmosphere. The expansion produces work that is generally put to use in driving a turbine (which serves, next to a steam turbine utilizing steam generated in the process, and/or a gas turbine), to drive one or more compressors. This typically refers to an air compressor employed in providing air to the oxidation process and/or to an NOx compressor; these compressors can be combined as a single apparatus, or e.g. be two different sections connected to a single driveshaft.In general, a nitric acid production process generates a high amount of process heat, and nitric acid production plants are set-up such as to make optimal use of the process heat, and in addition produce steam which can be exported to other plants on site. Thus, to the extent appropriate, heat exchange takes place between various gas streams, both of the nitric acid production circuit (i.e., process gases) and of an energy circuit (i.e., steam).        
It is desired in the art to further improve the energy usage in a nitric acid production plant. Also, it is desired to further reduce the content of nitrous gases vented into the atmosphere.